Where did our conventions about diagrams come from in the first place? 
Marshall mostly, for reasons people have suggested.

But if we ask why we do it now, the answer is surely tradition. Those of us 
(most?) who were taught a standard intro to economics course through most 
of the C20 (maybe a bit less towards the end of the century) have got those 
diagrams embedded in our brains (haven't  we?). Not just demand and supply, 
buit all the other diagrams with quantity on the horizontal axis. To try to 
change it would be to waste a large investment in human capital. Anyway, it 
saves a lot of effort if we all draw diagrams the same way round.

Tony Brewer
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